Let’s think of a MRI versus an X-ray.. A simple X-ray is all that is needed to find out if a bone is broken or not but an MRI is called for if one needs to go down to the tissue level to find out more serious problems..
Similarly , do we really need to do OTDR testing all the time for our fibre optic links ? The answer is No. A simple source and power meter test is more often than not enough to determine the attenuation on a fibre optic link and check if it is within the link budget specified for having a functional link.
Yes if we do have long haul fibre optic links running into a few tens of kilometers an OTDR trace will more accurately measure the attenuation to qualify the link not only in terms of optical link loss but also in terms of bends/kinks in the cable, inherent defects in the fibre (Macro & Micro bends).